Although a slot array antenna having a plurality of slots resonant with transmitted and received electromagnetic waves arrayed on side faces of a waveguide generally has low gain and high side-lobe level characteristics, a slot array antenna in which an aperture distribution of amplitude is made to be a desired distribution is disclosed in JP-A-H02-288708 to improve the characteristics.
Further, a parallel plate slot antenna in which a square-shaped parallel-plate waveguide is used with two slots as a unit of a radiation element to cancel out reflections from each other to suppress the reflections by the slots is disclosed in JP-B2-2526393.